Heretofore, studies have been aggressively made on the carbon fibers obtained by the vapor-growth process (vapor-grown carbon fibers) because carbon fibers having a high aspect ratio can be relatively easily obtained, and many proposals have been reported also on the production method therefor. The carbon nanotubes (that is, carbon fibers having a fiber diameter on the nanometer order) which are, in particular, recently attracting attention can also be synthesized by applying this vapor-growth process.
FIG. 1 is a schematic view showing one example of the reaction apparatus for continuously producing carbon fibers by the vapor-growth process. For example, in a general production method, carbon monoxide (CO), methane, acetylene, ethylene, benzene, toluene and the like are used as the starting material hydrocarbon. When the starting material hydrocarbon is a gas at an ordinary temperature, this is mixed, in the gas state, with a carrier gas and fed, whereas when it is a liquid, the starting material hydrocarbon is gasified, then mixed with a carrier gas and fed or is atomized in the liquid state in the heating zone. As the carrier gas, an inert gas such as nitrogen gas, a reductive hydrogen gas or the like is used. In some cases, the hydrocarbon is fed to a system depressurized to a vacuum. As the catalyst, a supported catalyst obtained by loading a metal on a support such as alumina, or an organic metal compound such as ferrocene is used. In the case of using a supported catalyst, the supported catalyst is previously disposed in the reaction zone and heated to perform a necessary pretreatment and, thereafter, the starting material hydrocarbon is fed and reacted (the case shown in FIG. 1), or the supported catalyst subjected to a pretreatment is continuously or intermittently fed, from outside the system, to perform the reaction. It is also possible to employ an organic metal compound readily dissolvable in the starting material hydrocarbon, such as ferrocene, as the catalyst precursor, continuously or intermittently feed it together with the starting material hydrocarbon to the heating zone, and produce a carbon fiber using, as the catalyst, the metal particle generated by the thermal decomposition of the catalyst precursor compound. The produced carbon fibers are collected in the inside of the heating zone and/or a collector 3 connected to the terminal of the heating zone at the terminal thereof and after the completion of reaction for a predetermined time, recovered.
The method for producing carbon fibers by the vapor-phase process is roughly classified into the following three types by the method of feeding a catalyst or a precursor compound for the catalyst:
(1) a method where a substrate or boat comprising alumina or graphite having supported thereon a catalyst or its precursor compound is placed in the heating zone and contacted with a hydrocarbon gas fed from the vapor phase;
(2) a method where particles of a catalyst or its precursor compound are dispersed in a liquid-state hydrocarbon or the like and continuously or pulsedly fed to the heating zone from outside of the system to contact with a hydrocarbon at a high temperature; and
(3) a method where a metallocene or carbonyl compound dissolvable in a liquid-state hydrocarbon is used as a catalyst precursor compound and a hydrocarbon having dissolved therein the catalyst precursor compound is fed to the heating zone, thereby contacting a catalyst and a hydrocarbon at a high temperature.
In the method (1), the steps of coating a catalyst or its precursor on a substrate, performing, if desired, a pretreatment such as reduction, producing carbon fibers and taking out the carbon fibers after lowering the temperature must be separately performed and this makes continuous production difficult and has poor productivity. On the other hand, in the methods (2) and (3), continuous production can be performed and high productivity is obtained. Therefore, in the industry, a method classified into (2) or (3) is employed. However, unless a great excess of a catalyst or precursor compound thereof is used as compared with the amount necessary for growth of the product carbon fibers, a sufficiently large amount of carbon fibers cannot be obtained. Thus, at present, an expensive catalyst or catalyst precursor compound is wasted and, moreover, a step of removing by-products originated in the catalyst added in an excess amount is provided. This is serious when the catalyst is not supported on a support or the like but a catalyst, produced by feeding a catalyst precursor such as ferrocene to the heating zone in the gas state or in the state of floating in the raw material gas, is used. The catalyst having high activity is considered to cause aggregation and become bulky, and, as a result, to lose the ability of growing a carbon fiber.
When an inorganic or non-aromatic carbon compound such as CO, methane, acetylene and ethylene is used as the carbon source, such a compound is low in the carbon fiber-producing rate and the method of (1) is used in many cases, but the contacting time between the catalyst and the carbon source compound is as large as from a few minutes to tens of minutes and the productivity is low.
When an aromatic compound such as benzene and toluene is used as the carbon source, continuous production can be performed by the method of (2) or (3), but as described above, a great excess of a catalyst or precursor compound thereof is necessary and the efficiency in effective use of the catalyst or its precursor compound is low, giving rise to a high cost.
In the production of carbon fibers by the above-described methods, one carbon compound is generally used as the carbon source. Although ferrocene or thiophene is added in many cases, these are each used as a catalyst precursor or a precursor of sulfur and not expected to play only a role of a carbon source. A very large number of patent publications disclose that two or more carbon compounds can be used as the carbon source, but those specifically describing the effect, and Examples of two or more carbon compounds being used, can hardly be found. There is absolutely no related art clarifying the scientific advantage regarding the use of two or more carbon compounds.
For example, Japanese Examined Patent Publication (Kokoku) No. 2521982 discloses that carbon fibers are produced by using an exhaust gas from a coke furnace, which is considered to be a mixture of various carbon compounds. However, the purpose thereof is to effectively use the exhaust gas and the scientific activity when these compounds are mixed is not referred to at all.
Japanese Unexamined Patent Publication (Kokai) No. 2003-81620 discloses that acetylene, ethylene or butadiene is mixed as a carbon compound with an aromatic compound. According to this publication, the temperature of reaction system can be elevated by adding acetylene, ethylene or butadiene, but Examples where such a carbon compound is actually mixed are not found and also, the composite effect obtained by the presence of two carbon compounds together is not referred to at all.
An object of the present invention is to provide a production method of vapor-grown carbon fibers, where the efficiency in effective use of a catalyst or a catalyst precursor is remarkably enhanced and, as a result, carbon fibers can be simply and effectively produced at low cost.